Have that questions about which card are accepted on sandbox for the payment i tried different mastercard and already have that same problem : " your card is invalid" i contact my bank and anything from them, to precise we are already on test and nothing is preleved it is only to continue integration tests
I can confirm we do accept mastercards as a payment option. Please see a list of test card details you can use to confirm.
As you’re adding a credit card to your sandbox account, no real transaction will be made. I would recommend double checking the long card number or the expiration date inputted.
I hope this helps and please let us know if you need any further assistance.